1. Field of the Invention
The present invention relates generally to active filters and, more particularly, relates to a switchable low pass filter having a multiple feedback topology.
2. Description of the Related Art
Active filters are well known in the art. A variety of circuit topologies exist for performing low pass, high pass and band pass functions. By the selection of circuit topology and component values, a wide range of frequency responses can be achieved.
FIG. 1 illustrates a general form of one such circuit topology, commonly referred to as an infinite gain, multiple feedback realization. FIG. 1A provides the generalized transfer function for this topology and FIG. 1B provides a table illustrating the specific component types required to realize various filter functions. By selecting the appropriate values for the admittances Y1 through Y5, a desired filter cut off frequency and second order filter response is attained.
In many applications, it is desirable to provide a filter which is dynamically switchable from a first frequency response to at least a second frequency response. This is often the case when signals of different frequencies need to be applied to the filter at different times. FIG. 2 illustrates a switchable second order low pass filter known in the prior art which uses the multiple feedback topology of FIG. 1A. Referring to FIG. 2, the circuit includes an operational amplifier A1 which functions as the active element. Signals are applied to the circuit through a first resistor R1 which is connected in series with a second resistor R2 and through R2 to an inverting input of the operational amplifier A1. At the junction of R1 and R2 is a first capacitor C1 which is connected in series with a second capacitor C2 which is connected to circuit ground.
A first electrically controllable switch S1 is coupled across the second capacitor C2. By operating switch S1, the total capacitance of the series circuit is altered. When the switch is in an open condition, the total capacitance is equal to the series combination of C1 and C2, which is stated as: EQU Ct=(C1*C2)/(C1+C2)
where Ct is the total capacitance of the series circuit. When the switch S1 is in a closed condition, the second capacitor C2 is bypassed and Ct=C1. Accordingly, when the switch is closed, Ct is a larger value which results in a lower frequency response of the filter.
The circuit further includes a third resistor R3 which is coupled in feedback from the output of the operational amplifier A1 to the junction of resistors R1 and R2. A third capacitor C3 is also included and is coupled from the output of the operational amplifier A1 to the inverting input of the operational amplifier A1. To complete the circuit, a series circuit including a second electrically controllable switch S2, a fourth capacitor C4 and a third controllable switch S3 is coupled in parallel with the third capacitor C3. When switches S2 and S3 are in an open condition, the total value of the feedback capacitance, Cf, is equal to the value of C3. When S2 and S3 are in the closed condition, C3 and C4 are connected in parallel. Therefore the value of Cf is increased and is expressed as: Cf=C3+C4.
Accordingly, by simultaneously opening S1, S2 and S3, the capacitance values Ct and Cf of the filter are decreased and result in a filter response with a first cut off frequency, F1. When the switches S1, S2 and S3 are all closed, the capacitance values Ct and Cf are increased, resulting in a second filter response with a second cut off frequency, F2, where F2&lt;F1.
While the circuit of FIG. 2 provides a selectable two-frequency response, this circuit topology has several disadvantages associated with the floating or flying configuration of the switches S2 and S3. Because of the floating configuration of the switches S2 and S3, at least a portion of the feedback signal is passed through these switches. Initially, it should be noted that typical electrically controllable switches deviate from that of ideal switches in that when "closed," the resistance is not zero ohms and when "open" the resistance across the switch is not infinite. In addition, the resistance of the switch can be modulated by the applied signal flowing through the switch. These factors result in signal distortion. Also, the switches S2 and S3, which generally take the form of CMOS analog switches, introduce significant parasitic capacitance to the circuit. Further, because the total feedback current is always flowing into the inverting terminal (virtual ground) of the operational amplifier regardless of the condition of the switches S2, S3, the Q and frequency response of this circuit is limited and good phase control without individual adjustment is only attainable at frequencies up to about 10% of the unity gain crossover frequency of the operational amplifier.